Various centerless or spindleless type rotary veneer lathes have been proposed which use a peripheral drive system together with a pair of spindles, or even dispense with the spindles, for applying turning torque to drive a peeler log in peeling wood veneer therefrom. In a veneer lathe without spindles, a peeler log is driven by the peripheral drive system which is designed specifically to apply the turning torque necessary to rotate the log on its axis from log's outer periphery. In a veneer lathe equipped with both the peripheral drive and the spindles, on the other hand, it is a usual practice that the log is supported and driven from its opposite axial ends by the spindles at least during an early period of the peeling operation and, at any suitable time during the peeling, the log drive is switched from the center drive spindles to the peripheral drive with the spindles then retracted from engagement with the log ends. In view of today's requirement of improvement in veneer yield and recent scarcity of large-diameter logs for veneer peeling, the veneer lathes of the above centerless type can offer important advantages to veneer and plywood mills in that small-diameter logs can be handled successfully and the final diameter to which a log can be peeled will not be restricted by diametral size of the spindles and hence the diameter of a core log resulting from veneer peeling can be reduced.
To peel veneer sheet with a predetermined thickness in a center-drive veneer lathe using conventional spindles, the feedrate, or the rate at which a veneer peeling knife carried on a movable knife carriage is fed into the log, is controlled in close relation to the spindle speed in terms of rpm. More specifically, the knife carriage is advanced for a predetermined distance, which equals the above predetermined thickness of veneer sheet to be peeled, for each turn of the log, hence each turn of the spindles. In a centerless type veneer lathe operation in which spindles, if any, are disengaged from the log at least during the later part of the veneer peeling operation and, therefore, the above feed controlling cannot be employed when the spindles are rendered inoperative, it has been customary to feed the knife carriage at a rate which is adjusted in accordance with the theoretical rotational speed of the log that can be figured out from the surface or peripheral speed of the log, which substantially equals the peripheral speed of the drive members of the peripheral drive system, and the current log diameter during peeling.
When the peripheral drive system uses rolls or discs as the drive members which are set in frictional engagement with the log periphery to drive the log, actual log speed may fail to coincide with its theoretical speed because of possible slippage between the drive members and the log periphery. Even with use of a peripheral drive system employing spiked wheels as the drive members, each having a number of radially projecting spikes incising into the log peripheral surface in driving the log, similar differences in peripheral speed between the log and the drive wheels may occur because of the varying incising depths of the spikes due to variations in hardness along the log surface. Insofar as the feed controlling is dependent on the above theoretical speed, not the actual speed, of a log, it is unavoidable in either type of drive that the feedrates of the knife carriage tend to become inaccurate, thereby to invite errors in the thickness of veneer sheet being peeled by a centerless type lathe.
Since it is practically impossible for a lathe operator to visually recognize the above slippage or the variations in relative speeds between the driving members and the log and also the variations in veneer sheet thickness caused by other various factors, the above problem inherent in centerless veneer lathes has been overlooked.